Chromatographic Separation of Vitamin E Enantiomers

Abstract

Vitamin E is recognized as an essential vitamin since its discovery in 1922. Most vegetable oils contain a mixture of tocopherols and tocotrienols in the vitamin E composition. Structurally, tocopherols and tocotrienols share a similar chromanol ring and a side chain at the C-2 position. Owing to the three chiral centers in tocopherols, they can appear as eight different stereoisomers. Plant sources of tocopherol are naturally occurring in the form of RRR while synthetic tocopherols are usually in the form of all-racemic mixture. Similarly, with only one chiral center, natural tocotrienols occur as the R-isoform. In this review, we aim to discuss a few chromatographic methods that had been used to separate the stereoisomers of tocopherols and tocotrienols. These methods include high performance liquid chromatography, gas chromatography and combination of both. The review will focus on method development including selection of chiral columns, detection method and choice of elution solvent in the context of separation efficiency, resolution and chiral purity. The applications for separation of enantiomers in vitamin E will also be discussed especially in terms of the distinctive biological potency among the stereoisoforms.

Keywords: Vitamin E; chromatography; enantiomers; tocopherols; tocotrienols.

Figure 1

Structures and methyl positions of tocopherols and tocotrienols.

Figure 2

Chiral separation of α-Toc methyl ether by Chiracel OD column. Peak 1: four 2S-isomers; Peak 2: RSS-; Peak 3: RRS; Peak 4: RRR; and Peak 5: RSR [28]. Reprinted with permission Copyright (1998) by Springer.

Figure 3

(a–e) Chromatogram of chiral separation of E/Z-α-tocotrienol methyl ether by Chiralcel OD-H column into their enantiomers: (a) enantiomeric pair of RS,Z-Z-α-tocotrienol methyl ether; (b) enantiomeric pair of RS,E/Z-α-tocotrienol methyl ether; and (c) enantiomeric pair of RS,E/Z-α-tocotrienol methyl ether. The last peak is presumed to be the degradation product of the secondly eluted enantiomer; (d) Peak 1: S,E-E-α-tocotrienol methyl ether; Peak 2: R,E-E-α-tocotrienol methyl ether; and Peak 3 is presumed to represent the degradation product of the Peak 2; (e) Chromatogram of chiral separation of RS,E/Z-α-tocotrienol methyl ether with omission of preparative HPLC step. Peak 1: diastereomeric pair of Z-Z- and E/Z-α-tocotrienol methyl ether; Peak 2: Z-Z-α-tocotrienol methyl ether enantiomer; Peak 3: diastereomeric pair of E/Z-α-tocotrienol methyl ether; Peak 4: S,E-E-α-tocotrienol methyl ether; Peak 5: R,E-E-α-tocotrienol methyl ether coeluted with a diastereomer of E/Z-α-tocotrienol methyl ether; and Peak 6 is presumed to be the degradation product of R,E-E-α-tocotrienol methyl ether [39]. Reprinted with permission. Copyright (2001) by Elsevier.

Figure 4

Gas chromatogram of all-rac-α-Toc-ME. The four pairs of diastereomers of all-rac-α-Toc-ME have equal peak height [44]. Reprinted with permission. Copyright (1981) by Hogrefe.

Figure 5

Schematic flow of the separation process for the eight stereoisomers.